Ethanol for industrial use is conventionally produced from petrochemical feed stocks, such as oil, natural gas, or coal, from feed stock intermediates, such as syngas, or from starchy materials or cellulosic materials, such as corn or sugar cane. Conventional methods for producing ethanol from petrochemical feed stocks, as well as from cellulosic materials, include the acid-catalyzed hydration of ethylene, methanol homologation, direct alcohol synthesis, and Fischer-Tropsch synthesis. Instability in petrochemical feed stock prices contributes to fluctuations in the cost of conventionally produced ethanol, making the need for alternative sources of ethanol production all the greater when feed stock prices rise. Starchy materials, as well as cellulosic material, are converted to ethanol by fermentation. However, fermentation is typically used for consumer production of ethanol, which is suitable for fuels or human consumption. In addition, fermentation of starchy or cellulosic materials competes with food sources and places restraints on the amount of ethanol that can be produced for industrial use.
As an alternative to fermentation, ethanol may be produced by hydrogenating acetic acid and esters thereof. Ethanol production via the reduction of acetic acid generally uses a hydrogenation catalyst. The reduction of various carboxylic acids over metal oxides has been proposed by EP0175558 and U.S. Pat. No. 4,398,039. U.S. Pat. No. 7,608,744 describes a process for the selective production of ethanol by vapor phase reaction of acetic acid at a temperature of about 250° C. over a hydrogenating catalyst composition either cobalt and palladium supported on graphite or cobalt and platinum supported on silica. U.S. Pat. No. 7,863,489 describes a process for the selective production of ethanol by vapor phase reaction of acetic acid over a hydrogenating catalyst composition to form ethanol is disclosed and claimed. In an embodiment of this invention, reaction of acetic acid and hydrogen over a platinum and tin supported on silica, graphite, calcium silicate or silica-alumina in a vapor phase at a temperature of about 250° C. U.S. Pat. No. 6,495,730 describes a process for hydrogenating carboxylic acid using a catalyst comprising activated carbon to support active metal species comprising ruthenium and tin. U.S. Pat. No. 8,309,772 discloses a process for selective formation of ethanol from acetic acid and includes contacting a feed stream containing acetic acid and hydrogen at an elevated temperature with catalyst comprising platinum and tin on a high surface area silica promoted with calcium metasilicate. U.S. Pat. No. 8,471,075 also discloses a process for selective formation of ethanol from acetic acid by hydrogenating acetic acid in the presence of first metal, a silicaceous support, and at least one support modifier. U.S. Pat. No. 6,204,417 describes another process for preparing aliphatic alcohols by hydrogenating aliphatic carboxylic acids or anhydrides or esters thereof or lactones in the presence of a catalyst comprising platinum and rhenium. U.S. Pat. No. 5,149,680 describes catalytic hydrogenation of carboxylic acids and their anhydrides to alcohols and/or esters in the presence of a catalyst containing a Group VIII metal, such as palladium, a metal capable of alloying with the Group VIII metal, and at least one of the metals rhenium, tungsten or molybdenum. U.S. Pat. No. 4,777,303 describes the productions of alcohols by the hydrogenation of carboxylic acids in the presence of a catalyst that comprises a first component which is either molybdenum or tungsten and a second component which is a noble metal of Group VIII on a high surface area graphitized carbon. U.S. Pat. No. 4,804,791 describes another production process of alcohols by the hydrogenation of carboxylic acids in the presence of a catalyst comprising a noble metal of Group VIII and rhenium. U.S. Pat. No. 4,517,391 describes preparing ethanol by hydrogenating acetic acid under superatmospheric pressure and at elevated temperatures by a process wherein a predominantly cobalt-containing catalyst is used and acetic acid and hydrogen are passed through the reactor, at from 210 to 330° C., and under 10 to 350 bar, under conditions such that a liquid phase is not formed during the process.
Hydrogenation catalysts having solid structures with layers have also been disclosed. US Pub. No. 2012/0296111 discloses an eggshell catalyst comprising an active metal selected from the group consisting of ruthenium, rhodium, palladium, platinum and mixtures thereof, applied to a support material comprising silicon dioxide, wherein the pore volume of the support material is 0.6 to 1.0 ml/g, determined by Hg porosimetry, the BET surface area is 280 to 500 m2/g, and at least 90% of the pores present have a diameter of 6 to 12 nm, to a process for preparing this eggshell catalyst, to a process for hydrogenating an organic compound which comprises at least one hydrogenatable group using the eggshell catalyst, and to the use of the eggshell catalyst for hydrogenating an organic compound. EP2583751 discloses heterogeneous catalysts useful for selective hydrogenation of unsaturated hydrocarbons, comprising palladium and optionally a promoter, supported on a substrate, having an uncoated BET surface area of ≦9 m2/g, the surface being coated with an ionic liquid. U.S. Pat. No. 8,211,823 discloses a selective hydrogenation catalyst, with alumina as carrier, and palladium as active component that distributed on the surface of the carrier in an egg-shell form, characterized in that: provided that the catalyst is weighed 100%, it comprises 0.2-0.5 wt % active component Pd, 2-8 wt % aids lanthanum and/or cerium, and 2-8 wt % alkaline earth metal. U.S. Pat. No. 7,375,049 discloses a catalyst suitable for the dehydrogenation and hydrogenation of hydrocarbons comprises at least one first metal and at least one second metal bound to a support material. The at least one first metal comprises at least one transition metal, suitably a platinum group metal. The support material is provided with an overlayer such that acidic sites on the support material are substantially blocked. U.S. Pat. No. 4,835,131 discloses a catalyst, which may have an egg-shell or an egg-yolk type distribution for the catalytically active component, and may be used in diffusion limited processes as well as in processes wherein the feedstock may contain catalyst poisons, e.g. in residue conversion.
Processes for producing the eggshell catalysts are described in U.S. Pat. No. 8,536,236, CN103100391, CN102451722, and CN102247856.
Thus, further improvements to hydrogenation catalysts to increase conversion of acetic acid and selectivity to ethanol are needed to improve production of ethanol from acetic acid.